A few months ago, from Astromol’s facebook, we posed a challenge: if we were able to reach four hundred followers, we would write more reports on stinky gases in the universe. And we got over them. So here are these “spatial flatulences”, today focused on hydrogen sulfide.
“Nature never ceases to amaze us.” As if it were the script of a documentary, I see myself pronouncing this phrase as I choose the title of this report. I know it’s a little scatological, but it’s totally true: let’s talk about some of the gases that are in our flatulence. Watch out, in our flatulence there’s not just gas, there are more things, but we’re not going to talk about those. The compounds that give farts that smell (not all smell the same, it will all depend on what we have ingested) are well known. And some of them are also in space. Now you’ll understand why I, who usually talk about Astrophysics and Astrochemistry, get into these issues.
Today we’re going to start a series of stinky gases with a gas that seems to be in a lot of places: hydrogen sulfide.
Tell me what you smell… and I’ll tell you what you’re made of
Olfaction is a sense to which we generally pay less attention than others such as sight or touch. However, it has been shown to have an important evocative capacity and its in-depth study earned a shared Medicine Nobel Prize in 2004. Regardless of what that might mean in terms of neural connections and olfactory memories, it’s clear that odors (unless we have a problem) catch our attention.
So, realizing that some of the gases we discovered in space, here on Earth smell terrible, we decided to get down to work and learn a little more about them. The surprise came when we compared the information and saw that many gases smell of something rotten (whether it’s eggs, fish or whatever). And, at the top of the eschatological, some of those gases detected in space are also present in flatulence.
Apparently, the worst in both worlds, the most nauseating, are hydrogen sulfide (which provides the “mixture” with a rotten egg-like smell) and carbonyl sulfide. They are joined by other sulfur compounds. They also contain methane, but keep an eye on appearances: methane has no smell at all, so we won’t discuss it in this report (just as we won’t talk about the problems humans suffer in their bellies when there’s excess gas).
Hydrogen sulfide (H2S) is a nasty piece of work with its good side produced in both biological and industrial processes. When in aqueous dissolution it is called hydrogen sulphide and, as gas, it is smelly, toxic, flammable, colorless and very stinky. It is usually in the environment in very, very small amounts (normal environmental content is 0.00011 to 0.00033 parts per million (ppm). As always, the quantity will determine the toxicity of a product.
Our protagonist is a gas somewhat heavier than air and is found naturally in volcanoes, in springs, in stagnant waters, drains, swamps, wells, sewers… In fact, if it bubbles up from the bottom of a lagoon and you’re unlucky enough to breathe it when it comes to the surface, you may stay in place forever. Just like if you enter a decaying fish warehouse that has been closed long enough to release it into the environment. If there is not too much (but very little, about 5 ppm) it will irritate your throat, eyes and entire respiratory system, but apparently, 20-50 ppm in the air is enough to cause you acute discomfort and, unlucky, drive you to the boatman’s arms.
Besides, hydrogen sulfide is a little misleading. According to this company that, among many other things, makes drilling, in large concentrations this gas becomes odorless… What a cheater. And what a deception for us who thought he was a loyal stinker. But wait… it’s not really that it loses its smelly character: it is that in large quantities it nullifies our olfactory capacity and we stop smelling it. If this happens, bad sign.
We could keep talking about hydrogen sulfide as a lethal agent, but let’s move on to reveal other things related to its discovery in space: it’s not all going to be bad, it also has other good things (or just characteristics inherent to it, in science we’re not manichean).
Discovery and transdisciplinarity
It was 1972 when P. Thaddeus, leading a five-person research team, discovered the presence of hydrogen sulfide in the interstellar medium. He used the 12-meter telescope at Kitt Peak National Observatory (USA). This work detected this compound in seven sources studied in our galaxy, the Milky Way . They estimated that the abundance of hydrogen sulfide was similar to that of formaldehyde, but had to wait until 2005 for Comito et al. to conduct an in-depth survey and determine its abundances.
By the way: the 1972 article was also signed by an A. A. Penzias and another R. W. Wilson. Do those names ring a bell? Six years later, both of them, working at Bell Telephone Laboratories, would receive the Nobel Prize for discovering, in 1965, the radiation from the cosmic microwave background. What does this have to do with astrochemistry? Well, a lot.
In 1950, Gerhard Herzberg published a book in which, without being too aware of it, he gave the keys to what would later become an impressive discovery: the echoes of the Big Bang, embodied in the radiation of the cosmic microwave background.
Many had predicted the temperature of the cosmic microwave background, but Herzberg gave the key when talking about a temperature of ‘2.30 K’, obtained from measuring the intensity of cyanide radical (CN). Regardless of this veiled discovery, he received a Nobel Prize in 1971, but not for this matter, but for his work related to free radicals, of which he determined his electronic structure and geometry. It is also known for being the main promoter of molecular spectroscopy, which today allows to study the properties and behaviors of molecules. We can say he was one of the fathers of modern Astrochemistry.
However, this detail about a certain residual temperature, which he mentioned in his work “Molecular spectra and Molecular structure”, went unnoticed, hidden behind his own commentary, defining it as something with a “restricted meaning”. As a result, it would be Penzias and Wilson who would make the definitive association between that temperature and the cosmic microwave backround.
In the words of José Cernicharo, researcher at CSIC, “had there been greater collaboration between astronomers and spectroscopists, this data would have caught the attention of the experts, advancing the discovery fifteen years”. Hence the importance of transdisciplinarity, collaboration between very different fields, to achieve together answers to so many questions. This is what is promoted from projects with Astromol or Nanocosmos, in which astrophysicists, astrochemists, laboratory experts, engineers, etc., come together for the same purpose: to know more about the chemistry of the universe.
But let’s go back to hydrogen sulfide.
As we said, it was P. Thaddeus who, along with his team, detected hydrogen sulfide in the interstellar medium in 1972. Nearly twenty years later, in 1991, Bockelee-Morvan and collaborators detected hydrogen sulfide at Austin’s comet and subsequently identified its presence on Halley’s comet . Recently, it has also been detected in Rosetta’s comet, 67P/Churiunov-Gerasimenco. 
The presence of hydrogen sulfide in comets can tell us about the formation of the comet’s own nucleus and the material that made up the nebula from which the Sun and the planets of the Solar System formed. That is, it gives us information about what materials were originally in that molecular cloud. An interesting fact from this work reveals that hydrogen sulfide detected in Halley’s comet is a parental compound.
Let’s explain: when a comet (an ice, rock and dust ball) heats up, the matter that makes it up starts to sublimate (it goes directly from solid state to gaseous state, without going through the liquid state). But this happens in several phases.
The primal matter that is solid in the comet, sublimates forming the coma (a cloud of gas and dust surrounding the comet). In this case we are talking about parental compounds that have sublimated directly from the comet without any chemical transformation. In the coma, however, parental compounds can undergo chemical transformations resulting in “daughter” species, which, having been processed, lose the informational value on the original composition.
Well, as we said, hydrogen sulfide in these comets seems to be “parental”, that is, it is there from the beginning of its formation.
Isn’t that amazing? Yes, ladies and gentlemen: the hydrogen sulfide of your flatulence, the one in comets, the one that swarms through the interstellar medium, comes from the same mother cloud. So, when they smell like rotten eggs and the person sitting next to you seems a little uncomfortable, think the universe is a set of wonders… (and that maybe the one who released the gas has had a bad day).
To be continued.
. The sources were: Orion A, W51 and in the DR21(OH) region.
 “From the intensity ratio of the lines with R = 0 and K = 1 a rotational temperature of 2.3º K follows, which has of course only a very restricted meaning”. From the book “Molecular spectra and Molecular structure”, Gerhard Herzberg.
 Eberhardt discovered the presence of H2S on Halley’s comet analyzing data from the Giotto probe, while Roesler et al. discussed the possible presence of hydrogen sulfide in Io, the Jupiter’s satellite.
 Organic molecules have also been detected, but none with sulfur. It is described in this work with Spanish participation: “Organic compounds on comet 67P/Churyumov-Gerasimenko revealed by COSAC mass spectrometry”.
Data about H2S and its discovery in different environments on the www.astrochymist.org webpage.
Originally published in Spanish on the Naukas website: Flatulencias espaciales (I), (2015/08/03).